The golden rule on the world wide web is that one should “never-ever-ever trust user input”. Given this, it would be wise to spend quite some time to make sure that all of the inputs from the users in able to make sure it is safe and what was expected..
There are several things to should watch out for when validating input:
Mistaken input.
For example the user types 99.7 rather than 9.97
Bad input
The user provides incorrect input in on purpose for whatever reason.
Dangerous input
User innocently enters information that would harm the system
Missing input
User provides no input.